Streaks - The to-do list that helps you form good habits.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Taskful - Deadlines, meet your match.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
ToDoList - Windows desktop application to stay on top of your daily tasks.
CherryTree - A hierarchical note taking application, featuring rich text and syntax highlighting, storing data in a single xml or sqlite file.